Polyurethane/urea elastomers are well-known and have wide use in the manufacture of a variety of products. The polyurethane/urea elastomers are formed by reacting a polyisocyanate with a composition having reactive hydrogen atoms and then chain extending, usually with a short chain reactant. Many systems are prepared by reacting an aromatic diisocyanate with a polyol, usually a long chain polyether or polyester diol, and then chain extending either with a short chain diol or aromatic diamine. Depending upon the reactivity of the aromatic diisocyanate and polyol, two techniques are widely used in the manufacture of such polyurethane/urea elastomers. One technique is a prepolymer technique wherein the aromatic diisocyanate is reacted with a long chain polyol and then chain extended with an aromatic diamine. Another is the reaction or liquid injection technique (RIM or LIM) wherein all of the reactants are mixed simultaneously, injected or cast into a mold and cured.
Representative patents which show the manufacture of polyurethane/urea systems include:
U.S. Pat. No. 3,285,879 which discloses the manufacture of polyurethane/urea systems by reacting an aromatic isocyanate with a long chain polyol and curing the prepolymer with a N-monalkyl aromatic diamine.
U.S. Pat. No. 4,374,210 discloses the manufacture of polyurethane systems by the RIM technique wherein an aromatic diisocyanate; namely methylene di(phenylisocyanate) is reacted with a long chain polyol and an alkyl-substituted aromatic diamine.
British patent 981,935 discloses a process for producing polyurethane-urea elastomers by reacting toluenediisocyanate with a long chain polyol, e.g., polytetramethylene glycol, to form a prepolymer and then reacting the prepolymer with a chemically hindered aromatic diamine.
To date, processes for producing solid form castable polyurethane elastomers are limited due to a variety of factors. First, many aromatic diisocyanates are toxic and exhibit high volatility. This toxicity factor coupled with high volatility requires special handling techniques. Second, the aromatic diisocyanates are reactive with aromatic diamines and the concentration and type of aromatic diamines used in the process are limited depending upon their particular reactivity with the isocyanate. Third, the isocyanate content in the prepolymer is fixed within a narrow range, and if additional isocyanate is added to the prepolymer to increase isocyanate content, reactivity may be too great or the resultant physical properties unacceptable for end use applications. Fourth, the viscosity of the resultant prepolymer is too high for the castable elastomer to be introduced into a mold. Attempts to reduce viscosity by the technique of heating often work to a disadvantage, particularly if aromatic diamine chain extenders are incorporated into the system, because of the increased reactivity at higher reaction temperatures. Higher temperatures also result in side reactions of the prepolymer to form allophanates.
Some of the problems noted above have been addressed but solutions have been limited. For example, there is literature illustrating various solutions to toxicity of aromatic diisocyanates by the reaction of the aromatic diisocyanate with short chain diols. However, the resultant product was a solid at polyurethane formation temperatures, and urethane systems utilizing such urethane linked diisocyanates required the use of solvents. Illustrative patents representing such processes for solving the volatility problem include the following:
U.S. Pat. No. 3,285,951 discloses the preparation of adducts of 2,4-toluenediisocyanate and 2,3-butanediol containing 85% meso isomer, with the mole ratio of toluenediisocyanate to butanediol being at least 2:1 preferably 2-4:1, for use in preparing polyurethanes. In one example, a product was prepared by dissolving toluenediisocyanate in a suitable solvent, such as hexane, stirring at high speed and adding butanediol to the solution. As the reaction was carried out, a solid white powder precipitated. The precipitate then was reacted with a millable gum of polypropylene glycol, butanediol and toluenediisocyanate.
U.S. Pat. No. 3,218,348 discloses the preparation of stable urethane polyisocyanate solutions by the sequential reaction of a polyisocyanate e.g., toluenediisocyanate, with a polyol mixture. To avoid crystallization on standing the polyisocyanate was reacted with a triol and then the resulting intermediate product was reacted with a diol. The patentees noted that when the procedure set forth in the 2,969,386 patent described above, was carried out adding the diol first and then the triol or alternatively, simultaneously adding the diol and triol to the isocyanate in the presence of organic solvent, the reaction product was unstable in the solution and would crystallize within a matter of minutes or days.
U.S. Pat. No. 3,076,770 discloses a process for producing low density cellular polyurethane foams by the reaction of an organic polyisocyanate with short chain polyols and utilizing the sequential addition of a triol and diol as in U.S. Pat. No. 3,218,348. Typically, high molecular weight polyols along with trifunctional polyols had been used for foams. The cellular polyurethanes were prepared by first reacting an isocyanate with a diol/triol mixture and removing free isocyanate from the reaction product. The reaction product was dissolved in a solvent and then reacted with a polyester polyol to form polyurethane systems.
U.S. Pat. No. 3,020,249 discloses a process for preparing rigid and semi-rigid polyurethane foams from an alkyd polyester resin and a diisocyanate containing reaction product derived from the reaction of toluenediisocyanate and 1,2,6-hexanetriol. The hexanetriol containing excess diisocyanate adducts were formed by reacting toluenediisocyanate with hexanetriol containing excess diisocyanate at temperatures of 100.degree.-120.degree. C.; the toluenediisocyanate was included in substantial excess.
U.S. Pat. No. 3,023,228 discloses a process for producing solid low molecular weight urethane polyisocyanate/urea systems by reacting a diisocyanate e.g., toluenediisocyanate, with a mixture of a short chain diol, e.g. butanediol, and water in the presence of a solvent, e.g., acetonitrile. Temperatures from about 10.degree.-35.degree. C. are suggested as being suited for forming the reaction product. An example shows reacting toluenediisocyanate with ethylene glycol in the presence of water and acetone (solvent) for about one to two hours at which time the reaction mixture solidifies. Another example described the reaction of toluenediisocyanate with diethylene glycol and water to produce a product having a softening point of 155.degree. C. and an isocyanate content of 23.5%. The resulting low molecular weight products having an isocyanate content typically from 18-25%, are valuable as reactants in the production of polyurethane plastics.